Most sites people go to are HTTP so it was decided to just not bother showing the protocol in newer versions of Firefox unless it's something else, like https:// or ftp://, thus saving a little space and making the URL a little more readable.
(note use of the passive voice)
You can set the pref browser.urlbar.trimURLs to false on the about:config page to see the http: protocol
In other words: if it shows nothing, it means it's the default http:
... at least until people start reporting exceptions.